4
Pin Descriptions The following pin function descriptions are for the 80C286 microprocessor.
SYMBOL
PIN
NUMBER
TYPE
DESCRIPTION
CLK
31
I
SYSTEM CLOCK: provides the fundamental timing for the 80C286 system. It is divided by two inside
the 80C286 to generate the processor clock. The internal divide-by-two circuitry can be synchro-
nized to an external clock generator by a LOW to HIGH transition on the RESET input.
D15 - D0
36 - 51
I/O
DATA BUS: inputs data during memory, I/O, and interrupt acknowledge read cycles; outputs data
during memory and I/O write cycles. The data bus is active HIGH and is held at high impedance to
the last valid logic level during bus hold acknowledge.
A23 - A0
7 - 8
10 - 28
32 - 43
O
ADDRESS BUS: outputs physical memory and I/O port addresses. A23 - A16 are LOW during I/O
transfers. A0 is LOW when data is to be transferred on pins D7 - D0 (see table below). The address
bus is active High and floats to three-state off during bus hold acknowledge.
BHE
1
O
BUS HIGH ENABLE: indicates transfer of data on the upper byte of the data bus, D15 - D8. Eight-bit
oriented devices assigned to the upper byte of the data bus would normally use BHE to condition chip
select functions. BHE is active LOW and floats to three-state OFF during bus hold acknowledge.
S1, S0
4, 5
O
BUS CYCLE STATUS: indicates initiation of a bus cycle and along with M/IO and COD/lNTA, de-
fines the type of bus cycle. The bus is in a TS state whenever one or both are LOW. S1 and S0 are
active LOW and are held at a high impedance logic one during bus hold acknowledge.
BHE AND A0 ENCODINGS
BHE VALUE
A0 VALUE
FUNCTION
0
Word transfer
0
1
Byte transfer on upper half of data bus (D15 - D8)
1
0
Byte transfer on lower half of data bus (D7 - D0)
11
Reserved
80C286 BUS CYCLE STATUS DEFINITION
COD/INTA
M/IO
S1
S0
BUS CYCLE INITIATED
0(LOW)
0
Interrupt acknowledge
00
0
1
Reserved
00
1
0
Reserved
0
1
None; not a status cycle
0
1
0
If A1 = 1 then halt; else shutdown
0
1
0
1
Memory data read
0
1
0
Memory data write
0
1
None; not a status cycle
1(HIGH)
0
Reserved
1
0
1
I/O read
1
0
1
0
I/O write
1
0
1
None; not a status cycle
11
0
Reserved
1
0
1
Memory instruction read
11
1
0
Reserved
1
None; not a status cycle
80C286